The first demonstration of entirely roll-to-roll fabricated perovskite solar cell modules under ambient room conditions.

The rapid development of organic-inorganic hybrid perovskite solar cells has resulted in laboratory-scale devices having power conversion efficiencies that are competitive with commercialised technologies. However, hybrid perovskite solar cells are yet to make an impact beyond the research community, with translation to large-area devices fabricated by industry-relevant manufacturing methods remaining a critical challenge. Here we report the first demonstration of hybrid perovskite solar cell modules, comprising serially-interconnected cells, produced entirely using industrial roll-to-roll printing tools under ambient room conditions. As part of this development, costly vacuum-deposited metal electrodes are replaced with printed carbon electrodes. A high-throughput experiment involving the analysis of batches of 1600 cells produced using 20 parameter combinations enabled rapid optimisation over a large parameter space. The optimised roll-to-roll fabricated hybrid perovskite solar cells show power conversion efficiencies of up to 15.5% for individual small-area cells and 11.0% for serially-interconnected cells in large-area modules. Based on the devices produced in this work, a cost of ~0.7 USD W-1 is predicted for a production rate of 1,000,000 m² per year in Australia, with potential for further significant cost reductions.

Direct synthesis of CsPbX3 perovskite nanocrystal assemblies.

Inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (NCs) possess many advantageous optoelectronic properties, making them an attractive candidate for light emitting diodes, lasers, or photodetector applications. Such perovskite NCs can form extended assemblies that further modify their bandgap and emission wavelength. In this article, a facile direct synthesis of CsPbX3 NC assemblies that are 1 µm in size and are composed of 10 nm-sized NC building blocks is reported. The direct synthesis of these assemblies with a conventional hot-injection method of the NCs is achieved through the judicious selection of the solvent, ligands, and reaction stoichiometry. Only under selective reaction conditions where the surface ligand environment is tuned to enhance the hydrophobic interactions between ligand chains of neighbouring NCs is self-assembly achieved. These assemblies possess narrow and red-shifted photoluminescence compared to their isolated NC counterparts, which further expands the colour gamut that can be rendered from inorganic perovskites. This is demonstrated through simple down-converting light emitters.

Precursor Engineering of Lead Acetate-Based Precursors for High-Open-Circuit Voltage Wide-Bandgap Perovskite Solar Cells.

Wide-bandgap (WBG) perovskites have great potential for inclusion in efficient tandem solar cells, but large open-circuit voltage losses have limited device performance to date. Here, we show that a high-quality WBG perovskite, FA0.83Cs0.17Pb(I0.8Br0.2)3, with enlarged grain sizes and improved crystallinity can be achieved by incorporating lead chloride (PbCl2) into a lead acetate (PbAc2)-based precursor. The improved film quality resulted in the suppression of nonradiative recombination and a reduction in defect density. Efficient WBG perovskite solar cells (1.66 eV) with an efficiency of 19.3% and a high Voc of 1.22 V were fabricated using a facile one-step spin-coating method without the need for an antisolvent. Notably, the unencapsulated devices retained 90% of their initial power conversion efficiency after storage in a dry box (10% humidity) for 800 h.

Enhanced Carrier Diffusion Enables Efficient Back-Contact Perovskite Photovoltaics.

Back-contact architectures offer a promising route to improve the record efficiencies of perovskite solar cells (PSCs) by eliminating parasitic light absorption. However, the performance of back-contact PSCs is limited by inadequate carrier diffusion in perovskite. Here, we report that perovskite films with a preferred out-of-plane orientation show improved carrier dynamic properties. With the addition of guanidine thiocyanate, the films exhibit carrier lifetimes and mobilities increased by 3-5 times, leading to diffusion lengths exceeding 7 µm. The enhanced carrier diffusion results from substantial suppression of nonradiative recombination and improves charge collection. Devices using such films achieve reproducible efficiencies reaching 11.2 %, among the best performances for back-contact PSCs. Our findings demonstrate the impact of carrier dynamics on back-contact PSCs and provide the basis for a new route to high-performance back-contact perovskite optoelectronic devices at low cost.

Omnidirectional Hydrogen Generation Based on a Flexible Black Gold Nanotube Array.

Solar-driven hydrogen generation is emerging as an economical and sustainable means of producing renewable energy. However, current photocatalysts for hydrogen generation are mostly powder-based or rigid-substrate-supported, which suffer from limitations, such as difficulties in catalyst regeneration or poor omnidirectional light-harvesting. Here, we report a two-dimensional (2D) flexible photocatalyst based on elastomer-supported black gold nanotube (GNT) arrays with conformal CdS coating and Pt decoration. The highly porous GNT arrays display a strong light-trapping effect, leading to near-complete absorption over almost the entire range of the solar spectrum. In addition, they offer high surface-to-volume ratios promoting efficient photocatalytic reactions. These structural features result in high H2 generation efficiencies. Importantly, our elastomer-supported photocatalyst displays comparable photocatalytic activity even when being mechanically deformed, including bending, stretching, and twisting. We further designed a three-dimensional (3D) tree-like flexible photocatalytic system to mimic Nature's photosynthesis, which demonstrated omnidirectional H2 generation. We believe our strategy represents a promising route in designing next-generation solar-to-fuel systems that rival natural plants.

High-Performance and Stable Semi-Transparent Perovskite Solar Cells through Composition Engineering.

Semi-transparent perovskite solar cells (ST-PeSCs) have tremendous potential as solar windows owing to their higher efficiency and visible transmittance. However, studies toward this application are still nascent, particularly in unraveling the interplay between how the perovskite composition impacts the achievable device performance and stability. Here, the role of A- and X-site modification in APbX3 perovskites is studied to understand their influence on these factors. Through detailed experimental and simulation work, it is found that a perovskite composition consisting of cesium (Cs) and formamidinium (FA) at the A-site delivers the best device performance over a range of band gaps, which are tuned by changes to the X-site anion. Using this optimized perovskite composition, power conversion efficiencies of 15.5% and 4.1% are achieved for ST-PeSCs with average visible transmittance values between 20.7% and 52.4%, respectively. Furthermore, the CsFA-based ST-PeSCs show excellent long-term stability under continuous illumination and heating. The stability of the precursor solutions across each of the studied compositions has also been considered, showing dramatic differences in the structural properties of the perovskites and their device performance for all mixed A-site compositions possessing the archetypal methyl ammonium species, while also confirming the superior stability of the CsFA precursor solutions.

Unconventional, Gram-Scale Synthesis of a Molecular Dimer Organic Luminogen with Aggregation-Induced Emission.

Organic luminogens have been widely used in optoelectronic devices, bioimaging, and sensing. Conventionally, the synthesis of organic luminogens requires sophisticated, multistep design, reaction, and isolation procedures. Herein, the products of the melt-phase condensation of benzoguanamine (BG; 2,4-diamino-6-phenyl-1,3,5-triazine) at 370-410 °C display interesting reaction-condition-dependent luminescence properties, including photoluminescence (PL) at a variety of wavelengths in the visible spectrum and quantum efficiencies (PLQE) of up to 58% in the powder form. With a simple and straightforward solvent washing procedure, the prominent blue luminescent component BG dimer was obtained in gram scale with >93% purity (96.5% purity after fractional sublimation). The BG dimer exhibited distinct aggregation-induced emission (AIE) properties. PL measurements indicate that the electronically excited state of the BG dimer undergoes efficient intramolecular nonradiative deactivation in room-temperature solution, leading to a significantly reduced PLQE (<0.1%) in solution. These nonradiative processes are substantially inhibited when the dimer existed in the form of crystals, solid aggregates in solution or being fixed in a rigid polymer film, resulting in a significant increase in the PLQE and lifetime. This work not only provided a new understanding for PL properties of self-condensation luminescent products but also represented an unconventional strategy for large-scale preparation of organic luminogens with high purity.

Detection of Halomethanes Using Cesium Lead Halide Perovskite Nanocrystals.

The extensive use of halomethanes (CH3X, X = F, Cl, Br, I) as refrigerants, propellants, and pesticides has drawn serious concern due to their adverse biological and atmospheric impact. However, there are currently no portable rapid and accurate monitoring systems for their detection. This work introduces an approach for the selective and sensitive detection of halomethanes using photoluminescence spectral shifts in cesium lead halide perovskite nanocrystals. Focusing on iodomethane (CH3I) as a model system, it is shown that cesium lead bromide (CsPbBr3) nanocrystals can undergo rapid (<5 s) halide exchange, but only after exposure to oleylamine to induce nucleophilic substitution of the CH3I and release the iodide species. The extent of the halide exchange is directly dependent on the CH3I concentration, with the photoluminescence emission of the CsPbBr3 nanocrystals exhibiting a redshift of more than 150 nm upon the addition of 10 ppmv of CH3I. This represents the widest detection range and the highest sensitivity to the detection of halomethanes using a low-cost and portable approach reported to date. Furthermore, inherent selectivity for halomethanes compared to other organohalide analogues is achieved through the dramatic differences in their alkylation reactivity.

Self-Assembly of Plasmonic Near-Perfect Absorbers of Light: The Effect of Particle Size.

Structures capable of perfect light absorption promise technological advancements in varied applications, including sensing, optoelectronics, and photocatalysis. While it is possible to realize such structures by placing a monolayer of metal nanostructures above a reflecting surface, there remains limited studies on what effect particle size plays on their capacity to absorb light. Here, we fabricate near-perfect absorbers using colloidal Au nanoparticles, via their electrostatic self-assembly on a TiO2 film supported by a gold mirror. This method enables the control of interparticle spacing, thus minimizing reflection to achieve optimal absorption. Slightly altering the nanoparticle size in these structures reveals significant changes in the spectral separation of hybrid optical modes. We rationalize this observation by interpreting data with a coupled-mode theory that provides a thorough basis for creating functional absorbers using complex colloids and outlines the key considerations for achieving a broadened spectral response.

Solution-processed antireflective coating for back-contact perovskite solar cells.

Back-contact architectures for perovskite solar cells eliminate parasitic-absorption losses caused by the electrode and charge collection layers but increase surface reflection due to the high refractive index mismatch at the air/perovskite interface. To mitigate this, a ∼85 nm thick layer of poly(methyl methacrylate) (PMMA), with a refractive index between those of air and perovskite, has been applied as an antireflective coating. Transfer matrix modelling is used to determine the ideal PMMA layer thickness, with UV-Vis spectroscopy measurements used to confirm the increase in absorption that arises through the application of the antireflective coating. The deposition of a thin film of PMMA via spin coating onto a solar cell results in a 20-30% relative increase in short circuit current density and stable power output density.

Facile purification of CsPbX3 (X = Cl-, Br-, I-) perovskite nanocrystals.

CsPbI3 perovskite nanocrystals are a promising optoelectronic material when stabilized in their cubic phase. While ongoing efforts have addressed this structural challenge through a variety of meta-stabilization approaches, the postsynthesis purification of these nanocrystal dispersions has remained a challenge. In this article, we undertake a detailed investigation into the chemical, optical, and structural changes that arise during purification of CsPbI3 nanocrystals. It is found that nanocrystal degradation can only be avoided through the judicious control of additives within each purification cycle. Under optimized additive-to-nanocrystal ratios, multiple purification cycles can be readily achieved, while retaining the quality and phase stability of the CsPbI3. This facile purification protocol ensures the preparation of high purity and high quality CsPbI3 nanocrystal inks that are suitable for better characterization or integration in optoelectronic devices. The approach has been generalized for CsPbX3 (X = Cl-, Br-, and I-).

Multiple Roles of Cobalt Pyrazol-Pyridine Complexes in High-Performing Perovskite Solar Cells.

Chemical doping is a ubiquitously applied strategy to improve the charge-transfer and conductivity characteristics of spiro-OMeTAD, a hole-transporting material (HTM) used widely in solution-processed perovskite solar cells (PSCs). Cobalt(III) complexes are commonly employed HTM dopants, whose major role is to oxidize spiro-OMeTAD to provide p-doping for improved conductivity. The present work discloses additional, previously unknown important functions of cobalt complexes in the HTM films that influence the photovoltaic performance. Specifically, it is demonstrated that commercial p-dopant FK269 (bis(2,6-di(1H-pyrazol-1-yl)pyridine) cobalt(III) tris(bis(trifluoromethylsulfonyl)imide)) reduces the interfacial recombination and alleviates the decomposition of the perovskite layer under the action of tert-butylpyridine and lithium bis(trifluoromethanesulfonyl)imide. These effects are demonstrated for 1 cm2 perovskite solar cells that achieve a stabilized power conversion efficiency of 19% under 1 sun irradiation.

Visualizing Phase Segregation in Mixed-Halide Perovskite Single Crystals.

Mixed organolead halide perovskites (MOHPs), CH3 NH3 Pb(Brx I1-x )3 , have been shown to undergo phase segregation into iodide-rich domains under illumination, which presents a major challenge to their development for photovoltaic and light-emitting devices. Recent work suggested that phase-segregated domains are localized at crystal boundaries, driving investigations into the role of edge structure and the growth of larger crystals with reduced surface area. Herein, a method for growing large (30×30×1 µm3 ) monocrystalline MAPb(Brx I1-x )3 single crystals is presented. The direct visualization of the growth of nanocluster-like I-rich domains throughout the entire crystal revealed that grain boundaries are not required for this transformation. Narrowband fluorescence imaging and time-resolved spectroscopy provided new insight into the nature of the phase-segregated domains and the collective impact on the optoelectronic properties.

Cadmium tris(dithiocarbamate) ionic liquids as single source, solvent-free cadmium sulfide precursors.

The first cadmium-based ionic liquids (ILs) have been developed, along with a family of cadmium dialkyldithiocarbamate salts, (cation)[Cd(R2dtc)3], in the pursuit of single source molecular precursors that thermolyse to form cadmium sulfide. Pyrrolidinium cadmium dialkyldithiocarbamate salts, (C4C1py)[Cd(R2dtc)3], salts were established to be ILs through thorough thermal and structural investigation.

An Alkylated Indacenodithieno[3,2-b]thiophene-Based Nonfullerene Acceptor with High Crystallinity Exhibiting Single Junction Solar Cell Efficiencies Greater than 13% with Low Voltage Losses.

A new synthetic route, to prepare an alkylated indacenodithieno[3,2-b]thiophene-based nonfullerene acceptor (C8-ITIC), is reported. Compared to the reported ITIC with phenylalkyl side chains, the new acceptor C8-ITIC exhibits a reduction in the optical band gap, higher absorptivity, and an increased propensity to crystallize. Accordingly, blends with the donor polymer PBDB-T exhibit a power conversion efficiency (PCE) up to 12.4%. Further improvements in efficiency are found upon backbone fluorination of the donor polymer to afford the novel material PFBDB-T. The resulting blend with C8-ITIC shows an impressive PCE up to 13.2% as a result of the higher open-circuit voltage. Electroluminescence studies demonstrate that backbone fluorination reduces the energy loss of the blends, with PFBDB-T/C8-ITIC-based cells exhibiting a small energy loss of 0.6 eV combined with a high JSC of 19.6 mA cm-2 .

Aqueous Synthesis of High-Quality Cu2ZnSnS4 Nanocrystals and Their Thermal Annealing Characteristics.

Copper zinc tin sulfide (CZTS) nanocrystal inks are promising candidates for the development of cheap, efficient, scalable, and nontoxic photovoltaic (PV) devices. However, optimization of the synthetic chemistry to achieve these goals remains a key challenge. Herein we describe a single-step, aqueous-based synthesis that yields high-quality CZTS nanocrystal inks while also minimizing residual organic impurities. By exploiting simultaneous redox and crystal formation reactions, square-platelet-like CZTS nanocrystals stabilized by Sn2S64- and thiourea are produced. The CZTS synthesis is optimized by using a combination of inductively coupled plasma analysis, Raman spectroscopy, Fourier transform infrared spectroscopy, and synchrotron powder X-ray diffraction to assess the versatility of the synthesis and identify suitable composition ranges for achieving phase-pure CZTS. It is found that mild heat treatment between 185 and 220 °C is most suitable for achieving this because this temperature range is sufficiently high to thermalize existing ligands and ink additives while minimizing tin loss, which is problematic at higher temperatures. The low temperatures required to process these nanocrystal inks to give CZTS thin films are readily amenable to production-scale processes.

Dipole-field-assisted charge extraction in metal-perovskite-metal back-contact solar cells.

Hybrid organic-inorganic halide perovskites are low-cost solution-processable solar cell materials with photovoltaic properties that rival those of crystalline silicon. The perovskite films are typically sandwiched between thin layers of hole and electron transport materials, which efficiently extract photogenerated charges. This affords high-energy conversion efficiencies but results in significant performance and fabrication challenges. Herein we present a simple charge transport layer-free perovskite solar cell, comprising only a perovskite layer with two interdigitated gold back-contacts. Charge extraction is achieved via self-assembled monolayers and their associated dipole fields at the metal-perovskite interface. Photovoltages of ~600 mV generated by self-assembled molecular monolayer modified perovskite solar cells are equivalent to the built-in potential generated by individual dipole layers. Efficient charge extraction results in photocurrents of up to 12.1 mA cm-2 under simulated sunlight, despite a large electrode spacing.Simplified device concepts may become important for the development of low cost photovoltaics. Lin et al. report solar cells based on interdigitated gold back-contacts and metal halide perovskites where charge extraction is assisted via a dipole field generated by self-assembled molecular monolayers.

Alternating 5,5-Dimethylcyclopentadiene and Diketopyrrolopyrrole Copolymer Prepared at Room Temperature for High Performance Organic Thin-Film Transistors.

We report that the inclusion of nonaromatic 5,5-dimethylcyclopentadiene monomer into a conjugated backbone is an attractive strategy to high performance semiconducting polymers. The use of this monomer enables a room temperature Suzuki copolymerization with a diketopyrrolopyrrole comonomer to afford a highly soluble, high molecular weight material. The resulting low band gap polymer exhibits excellent photo and thermal stability, and despite a large π-π stacking distance of 4.26 Å, it demonstrates excellent performance in thin-film transistor devices.